# 15: Ribozymes, Enzyme Kinetics

- Page ID
- 6105

**Reading& Problems**: LNC p. 200-206; p. 238 prob. 8, 11, 13; Segel, p.319, prob. 1,2,3

### I. Enzyme Kinetics - measuring/calculating the Velocity of an enzyme catalyzed reaction:

**Rate of production of product is the velocity V = d[**P**]/dt**

### A. Assumptions

**Initial rate**- we are in the initial phase of the reaction when the reaction has proceeded to a small enough extent that the [**S**] does not change significantly.**Steady state**- the [**ES**] rapidly reaches a steady state that does not change over the time being analyzed.

### D. Plotting enzyme data (here is a link to download the enzyme plots used in class)

- [
**P**] vs. time - V versus [
**S**]

### A. Lineweaver-Burk double reciprocal plot: 1/V vs 1/[**S**] and determination of K_{m} (X-intercept is -1/K_{m}) and V_{max} (Y-intercept is 1/V_{max}), slope is K_{m}/V_{max}

### B. The meaning of the constants

- K
_{m}- the [**S**] that leads to half maximal velocity, a measure of the affinity of the enzyme for the substrate. Range 0.4 µM - 10 mM. Lower K_{m}means higher affinity. - V
_{max}- not really a constant, =k_{cat}[E_{t}] and [E_{t}] can vary. It is only a constant at a constant enzyme concentration. - K
_{cat}- how many reactions one enzyme can perform per unit time. In inverse time units. In sec^{-1}(per second) called "turnover number". Range from less than 0.5 up to 6 x 10^{5}. Higher K_{cat}means faster reaction. - Best measure of enzyme efficiency is K
_{cat}/K_{m}. Bigger number is more efficient enzyme.

**Some take home information:**

Lineweaver-Burk double reciprocal plot: 1/V vs 1/[**S**] can be used to determine K_{m} (X-intercept is -1/K_{m}) and V_{max} (Y-intercept is 1/V_{max})

### Contributors

Charles S. Gasser (Department of Molecular & Cellular Biology; UC Davis)